ApoD has been isolated in electrophoretically homogeneous form from HDL by affinity and gel permeation chromatography. ApoD contains carbohydrate and several disulfide bridges by amino acid analysis. A higher molecular weight form of apoD was also isolated, and shown to be a apoD-A-II dimer. ApoD has recently been proposed to be the cholesterol ester exchange protein which is involved in the exchange of cholesterol ester between plasma lipoproteins. ApoE has been isolated in electrophoretically homogeneous form from normal subjects and patients with Type III and Type V hyperlipoproteinemia. The amino acid composition of apoE from all subjects was similar by amino acid analysis. On isoelectric focusing apoE from Type III patients was missing the E3 band. ApoG, a newly discovered apolipoprotein, was purified from thoracic duct lymph and plasma of patients with Type V hyperlipidemia. The apolipoprotein has a unique amino acid composition, is a glycoprotein and contains several disulfide bridges. The physiological role of apoG on lipoprotein metabolism is as yet unknown.